Radical new laws should allow police to positively discriminate in favour of minority ethnic recruits, otherwise the ranks of officers will be too white for decades to come, the leader of Britain’s police chiefs has said.

In an interview to mark the 20th anniversary of the Macpherson report into the death of Stephen Lawrence, Sara Thornton said police had made huge progress, but “unconscious bias” still existed and the way they used their powers needed to be seen to be fair.

Thornton is the chair of the National Police Chiefs’ Council and will retire next month after 33 years of service.

She rose to the top having been part of the Metropolitan police team during the Macpherson inquiry, a watershed for policing which still shapes public policy and debate today.

Thornton said new laws should be passed to “shock the system”.

Since 1999 police have been trying to get the proportion of officers from ethnic minorities to match the proportion in the populations they serve.

Thornton said her personal view was that positive discrimination was needed: “That is unlawful at the moment. If you want to do something to give a shock to the system and say we can’t wait to 2052, I think we need to do something different.

“It is a political judgment, isn’t it? How important is this? If it’s important, then I think you need to look at a different approach.”

While 14% of the population are from an ethnic minority, just 7% of police in England and Wales are – up from 2% when Macpherson reported 20 years ago.

John Grieve: The Macpherson report left a profound and lasting legacy

Read more

Thornton said a quarter or even 30% of new recruits in some big forces were from an ethnic minority. But changing the overall makeup of forces was slow and budget cuts meant few new officers joined between 2010-2015.

“It will take a long time. The turnover of police officers is really quite slow, so it is about 6% a year, it’s always going to take you a long time, and it’s about whether we can wait,” she said.

Police chiefs such as the former Metropolitan police commissioner Lord Hogan-Howe have favoured the move, but the government has balked at the idea of introducing positive discrimination on the grounds of race. It has been used in Northern Ireland to get more Catholics into the police service and in the US.

Positive action is legal and allows measures to encourage applications from under-represented groups. Some chiefs believe the focus on race issues has been lost at times, so more could be done without a fight with the government to change the law.

Thornton said: “I think there’s an argument that we could select on merit and put people into a pool [of recruits] and then appoint on representation.”

Thornton was once described as the former prime minister David Cameron’s favourite police chief. She is a former chief constable of Thames Valley, which covers the constituency of Theresa May, who pressed the police over race while home secretary.

Thornton, 56, is expected to be announced on Friday as the government’s new anti-slavery commissioner.

Thornton said “unconscious bias” still existed in policing and there were problems in promotion, and minority ethnic officers being more likely to face disciplinary action.

“If we like ourselves, we prefer people who are like ourselves, because we understand them and they are familiar. People who aren’t like ourselves, sometimes we feel less comfortable with. What are those stereotypes we sometimes just fall into?” She added that unconscious bias training had been introduced to combat this.

“I do think we can have policies and procedures that can unintentionally discriminate.”

She gave the example of one large force, Greater Manchester, holding a promotion board during Ramadan, when Muslim candidates would be fasting and thus possibly physically weaker and at a disadvantage.

Thornton said she did not like to use the term institutional racism – the headline finding of the Macpherson inquiry – because it was misunderstood and taken as a slur on every officer.

Thornton said forces needed to treat their officers fairly, and they in turn needed to be seen to use their powers justly. It was crucial for legitimacy in a service that prided itself on “policing by consent”, she said, with the issue being even more important because police exercised the “tyranny and majesty of state power”.

Thornton said: “For marginalised communities, policing can represent the tyranny of state power.

“Let’s be candid about this, if within communities there is a sense of being over-policed and under-protected, it’s not the sort of occupation parents are going to encourage their youngsters to get involved in.”

Police had made huge progress, she said, with the gap in black people’s trust in officers narrowing to 7% less than white people’s.

Stephen Lawrence, 18, was murdered by a white racist gang at a south-east London bus stop in 1993 and Macpherson found police incompetence and prejudice, which he described as “institutional racism”, helped the killers escape justice.

Police fought suggestions of errors and bias with vehemence, but were exposed by the inquiry. Police chiefs were the last to realise the injustice their force had inflicted on the murdered victim’s parents, Doreen and Neville Lawrence.

Thornton, a superintendent when the inquiry held its dramatic hearings, said Met leaders who had given their working life to the force could not believe the Lawrence investigation had gone so wrong, before the Macpherson inquiry laid the failings bare.

Thornton said an argument from some officers that police prejudice was no worse than that of others in society was not good enough: “We are police officers with significant powers and therefore it matters much more.”

• This article was amended on 22 February 2019. Sara Thornton was a superintendent at the time of the Macpherson inquiry, not an inspector.